Cranes
have eyelids that move from front to back instead
of up and down like our eyelids.It
is actually
the crane's nictitating
membrane—or inner eyelid, and the crane's actual
eyelid closes
from the bottom to the top.
You can see the crane's eye through the closed nictitating membrane,
and the crane can see through it, too. Scroll down to learn more.

Photo:
Eva Szyszkoski

Birds
need to be able to react to things quickly—literally in
the blink of an eye—while flying, and when cranes duck
their heads into water to feed, the nictitating membrane can
protect
their eyes from particles while they grab food. Most of the time
when a bird blinks, it is closing the nictitating membrane to
keep the eyeball moist and clear of dust and junk. But birds
occasionally blink with the outer eyelid to keep the nictitating
membrane moist and clean.
When birds sleep they close the outer eyelid. What
might be the reason?